Clutches are well known devices used to selectively connect a source of rotational power, such as the crankshaft of an internal combustion engine and its flywheel, to a driven mechanism, such as a transmission. Typically, clutches have a driven disc rotatably fixed to the transmission input shaft and axially disposed between a flywheel and a pressure plate. Both the flywheel and the pressure plate are rotatably fixed to the output shaft of the engine. The pressure plate is axially biased toward the flywheel by an axial spring load. When the clutch is in an engaged condition, the pressure plate clamps the driven disc against the flywheel. Friction material is disposed on both sides of the driven disc to resist slipping between the driven disc and both the pressure plate and the flywheel. When the clutch is in a released condition, the axial spring load is overcome by a release mechanism, unclamping the driven disc. With the driven disc unclamped, relative rotation between the transmission input shaft and the engine output shaft becomes possible. When the clutch is reengaged, the pressure plate is pressed against the friction material, halting relative rotation between the engine output shaft and the transmission input shaft.
When the clutch is reengaged, and to a lesser degree when the clutch is released, the friction material wears due to the contact at relative speed with the pressure plate and flywheel.
Commonly, the friction material on the driven disc is provided in the form of a plurality of discrete elements or cookies. The cookies are adhesively bonded or brazed to metal plates to form friction material buttons. The buttons are in turn fixed to radially extending paddles of the driven disc assembly by rivets which pass through laterally extending flanges of the backing plates overlying the paddles. The thickness of the rivet heads limits the amount of the friction material available for wear which can be usefully employed to provide engagement between the engine and the transmission. To compensate for the rivet head thickness, the friction material is made thicker than would otherwise be necessary. Also, the backer plate and the disc paddles are both larger than the cookies to enable the buttons to be riveted to the paddles at their outer edges.
Disadvantages of riveting the buttons to the paddles include: the need to provide the necessary extra thickness of friction material for clearing the rivet heads and the associated increased rotational inertia contributed by the friction material; the extra rotational inertia attributable to the extra backer plate material and extra disc material used at the rivet locations; and a susceptibility to warpage of the cookie, characterized by the center or edges of the friction material lifting off of the backer plate with usage of the clutch and wear of the friction material.
It is desired to provide a driven disc with a reduced height attachment for friction material buttons which alternatively enables the use of thinner friction material cookies or extended wear of the friction material. It is also desired to provide a driven disc assembly having lower inertia. It is also desired to provide a driven disc having the buttons mounted thereto in such a manner that the tendency of the cookies to distort relative to the driven disc is reduced.
It is also desired to provide a method of making a driven disc having a reduced height attachment for friction material buttons which enables the use of thinner friction material cookies, or, alternatively, enables the extended wear of the friction material. It is also desired to provide a method of making a driven disc having lower inertia. It is also desired to provide a driven disc having buttons mounted thereto in such a manner that any tendency of the cookies to distort relative to the driven disc is reduced.